Traditionally, production of crude vegetable oils has been accomplished by mechanical pressing or solvent extraction, and, in some cases, by a combination of both, especially for seeds with high oil content. )On the average, most crude oils consist of over 95% triglyceride molecules, while minor components are present in the form of free fatty acids (FFA), phospholipids (gums), color pigments, moisture and proteinaceous materials. Thus in order to obtain vegetable oils with acceptable industrial and/or edible quality, the crude oil is often subjected to a series of processes designed to remove the various impurities. Such processes include: degumming, refining (deacidification), bleaching, winterization and deodorization.
Of the various refining processes, removal of FFA (deacidification) is deemed most critical due to the fact that high level (&gt;0.5%) of FFA could reduce the shelf-life of the oil by promoting rancidity, moreover, downstream processing is adversely impacted by free fatty acids impurity. Over the years, the most industrially accepted deacidification method is caustic (alkaline) refining, a process during which the crude oil is treated with dilute solution of sodium hydroxide. In a typical alkaline refining process, the oil is passed into a tank and then thoroughly mixed with sodium hydroxide solution (concentration depends on the level of FFA in the crude oil) at 70.degree. C. to 80.degree. C. for about 1-15 minutes. The resulting slurry is then centrifuged in order to eliminate the soapstock, and the oil is washed with generous amount of water to further remove traces of soapstock. For continuous processes, a series of centrifuges are used to separate neutral oil from soapstock, after which the refined oil is passed through a vacuum dryer to remove residual moisture.
A modification of the continuous alkaline deacidification process is the so called "miscella refining", in which crude oil, dissolved in a solvent (usually hexane), is treated with alkaline solution, then thoroughly mixed, and subsequently centrifuged to separate soapstock and refined miscella (oil is still dissolved in solvent). The alkaline concentration ranges from 4% to 15% depending on the amount of free acid and the other impurities in the crude oil. Miscella refining requires very intensive plant management and high capital investment since most of the machinery must be explosion-proof, and the processes are performed at greater than ambient temperatures.
Regardless of the type of alkaline refining, the process is well established, however, there are many problems associated with it. Treating a triglyceride oil with sodium hydroxide at moderately high temperature would result in some degree of saponification, resulting in a loss of neutral oil. There may also be an emulsion problem during water treatment which often necessitates the use of emulsifying agents. In the case of miscella refining, there can be fouling of the distillation system by residual soapstock, and high labor cost and capital investment. The current processes result in appreciable loss (&gt;5%) of neutral oil, which increases the operating cost.
Consequently, there is a need to develop alternative methods for deacidification of crude vegetable oils. To a limited extent, physical refining has been used for deacidification. In such a process, the oil is treated with steam at elevated temperature in order to steam distil the free fatty acids. However, steam distillation is less efficient than alkaline refining. Moreover, the physical refining process may not be applicable to most oils, due to the varying affinities of different molecules for co-distillation with water.
Recently, many other deacidification processes have been reported, some of which include: biological refining, in which bacteria species are allowed to feed on the FFA; fractionation using furfural as a polar solvent to selectively remove the free acids. Thus, there are continuing research programs aimed at developing efficient, and yet practical modes of solving the deacidification problem facing the vegetable oil industry.